Power converters for medium and high voltage applications are commonly used for converting a first current at a first frequency and a first voltage into a second current at a second frequency and a second voltage. Many types of different power converters are known, such as for converting AC to AC, AC to DC, DC to AC and DC to DC.
A switching regulator is a circuit that includes a controller and a power converter including at least one power phase (or stage) that can comprise a power switch, an inductor, and a diode, a high-side power transistor and a low-side power transistor, or 4 power switches with the inductor between the respective pairs of switches for a Buck-boost converter. The power stage is connected in series between supply terminals with a converter switching node at the interconnection of the power transistors adapted for connection to the inductor to transfer energy from the input to the output, where the power switch(es) converts the input voltage to the desired output. The controller and power phase(s) are linked by a feedback loop, and the controller supervises the switching operation of the power phase(s) to regulate the output voltage.
The basic components of the power phase(s) can be rearranged to form a switching converter such as for example as a buck, boost, or buck-boost converter to regulate the energy transfer and maintain a constant output voltage within normal operating conditions. In the simplest case of a single energizing and de-energizing phase it can be established that for a given inductor charging time, tON, and a given input voltage and with the converter circuit in equilibrium, there is a specific inductor discharge time, tOFF, for a given output voltage.
When designing a switching DC-DC regulator system to predict the length of the different operating phases (being at least one charging time and at least one discharging time) during each switching cycle constituting the pulse-width modulation (PWM) or PFM regulation. This allows for example one to design a PFM inductor current pulse comprising an energizing pulse where the inductor current increases and a de-energizing pulse where the inductor current decays (decreases) back to zero.
The energizing and de-energizing pulses should relate to each other according to the volt-second balance principle so that the end of the de-energizing pulse (associated with rampdown in inductor current) happens when the inductor current returns to the same current value so that steady-state operation is reached.
Regarding the meaning of volt-second balance, the voltage (V) across an inductor of a power phase of a power converter is V=L*di/dt, where L is the inductance of the inductor and i is the inductor current, and t is time, so that L*di=V*dt. The change in inductor current di is thus proportional to the product of the inductor voltage V and dt. As long as a constant polarity of voltage across the inductor is applied over a continuous period of time, di is non-zero, and hence the current across the inductor keeps ramping in magnitude one direction. In the simplest case of a single energizing and de-energizing phase, for the inductor current to reach steady-state operation over time, if the current ramps up by di over time t1, it must ramp down by di over time t2. Only then would the inductor current have returned to its initial state after a time of t1+t2.
When a switching regulator designer is not able to predict the inductor current rampup (during the energizing operating phase) and inductor current rampdown (during the de-energizing operating phase) times accurately during the switching cycles, conventionally the designer needs to rely on current comparators which are used for checking the reversal of the inductor current and the controller's stopping of the de-energizing phase. These current comparators are difficult to design because they need to be fast in the control mode (e.g., PFM mode) where the quiescent current is low. The propagation delay of the current comparators also results in a timing error added on top of their DC offset error.